1. Field of the Invention
The present invention relates generally to a steering system for vehicles. More particularly, the invention relates to a steering system for a vehicle with a front wheel and a rear wheel, of the type in which the rear wheel is steerable in relation to a steering operation of the front wheel, especially such that the rear wheel has a steered angle ratio thereof relative to the front wheel controlled to be variable.
2. Description of Relevant Art
There has already been proposed, in Japanese Laid Open Application No. JP-A-59-26364, a steering system for a vehicle with a front wheel and a rear wheel, in which the rear wheel is steerable in relation to a steering operation of the front wheel, such that a steered angle ratio of the rear wheel to the front wheel is controlled to be variable in accordance with a travelling speed of the vehicle.
More particularly, in such steering system, at relatively low vehicle speeds the rear wheel is steered in the opposite direction to the front wheel or substantially at an angle of zero degree and, at relatively high vehicle speeds, in the same direction as the front wheel. With reference to FIG. 5 of the accompanying drawings, the steered angle ratio is given as a continuous function of the vehicle speed, as represented by a curve Q.sub.3 :.gamma.=f.sub.3 (V), such that for an arbitrary vehicle speed V higher than a predetermined vehicle speed d.sub.1, there is given a positive angle ratio .gamma., i.e., the front and rear wheels are steered in the same direction. For an arbitrary vehicle speed V lower than the predetermined vehicle speed d.sub.1, there is given a negative angle ratio, i.e., the front and rear wheels are steered in opposite directions to each other, whereby a continuous control of the steered angle ratio is attained, ranging from a low speed to a high speed. As a result, at relatively low vehicle speeds, the minimum turning radius of the vehicle as well as the inside wheel span is remarkably reduced, and in addition the vehicle turning characteristics such as when parking, travelling along a narrow curved road, or making a U-turn, are improved. Moreover, at relatively high vehicle speeds, the steering responsiveness can be greatly improved.
In a vehicle with such a steering system, a rear wheel is automatically controlled so as to have a steered angle ratio thereof to a front wheel determined through a given function f.sub.3 (V) in correspondence with a vehicle speed V.
However, under particular conditions such as on an expressway, which is designed substantially straight, small-turn operations are seldom required, i.e., only relatively infrequent lane changes are required, even when the vehicle is forced to low-speed travel by traffic jams and the like.
In this respect, in a vehicle with a steering system of the above-mentioned type, the driver will be afforded greater convenience if the steering system is improved to be intentionally flexible and voluntarily changeable with respect to the steering characteristics, in accordance with subjective factors such as the preference or physical condition of the driver while travelling, so that the steered angle ratio can be set, at low speeds of the vehicle, to be negative as in the conventional case, permitting very tight turns such as in a U-turn operation; and, at medium and slightly higher speeds, not only positive, but rather somewhat negative, to thereby provide the steering characteristic with a sporty feel.
The present invention provides an improvement in a steering system for vehicles of the above-mentioned type.